Firefly and PC GAMESS-related discussion club

I thank you for your answer, but I think that the problem lies elsewhere. At the CAS level of theory I calculate singlets and triplets together. At the XMCQDPT2 I have TWO DIFFERENT calculations with TWO DIFFERENT effective Hamiltonians (for singlets and for triplets).For example, if I take 10 and 10 states for singlets and triplets, I have for singlets:

>I'm not Alex, but I'll try to help. First of all, CASSCF state order is not guaranteed to be the correct one=)>Larger effective Hamiltonian dimension should yield more correct results, however for big systems it is simply impractical (high computational effort) to calculate Heff for more than 10-20 states. 2x2 seems to be a bit extreme in the low end because higher states (up to 5 or even more sometimes) can be of influence to the lowest ones. I would recommend to conduct a single large calculation to see the "EIGENVECTORS OF THE EFFECTIVE HAMILTONIAN".>For example for one of my systems:> 1 2 3 4 5

>This means that to correctly describe state 3 it is crucial to include state 4. This can also be seen in the CAS-CI states, states 3 and 4 share some large weighted CSF-S

>Best regards, Igor

>On Fri Dec 23 '16 5:37pm, Maksim Shundalau wrote>------------------------------------------------>>Dear Alex,>>>>>>Let's pretend that I calculated two lower states (triplet and singlet) of transient molecule (or radical) at the CASSCF level of theory. I get triplet as a ground state and singlet is only a few tens of cm-1 higher than the triplet. >>How many triplets and singlets should I take in the XMCQDPT2 procedure to get the same sequence of states and trust the results? If I take 2 and 2 (or 10 and 10), I have triplet higher than singlet. Should I take 3 and 2 (or 5 and 2, for example) for triplet and singlet, respectively? >>>>>>Kind regards,>>Maksim Shundalau.>>